Electroplating substrates with copper is generally well known in the art. Electroplating methods involve passing a current between two electrodes in a plating solution where one electrode is the article to be plated. A common plating solution is an acid copper plating solution comprising (1) a dissolved copper salt (such as copper sulfate), (2) an acidic electrolyte (such as sulfuric acid) in an amount sufficient to impart conductivity to the bath, and (3) various additives such as surfactants, brighteners, levelers and suppressants, to enhance the effectiveness of the bath.
“Electroforming” refers to the process of electrodepositing a metal (such as copper) on a mandrel to produce an independent, mechanically viable, metal object that can stand alone when separated from the mandrel. Various metals can be electroformed, including, for example, copper, nickel, iron and various alloys thereof. The metal is electrodeposited on the mandrel to a desired thickness and the mandrel is then removed to separate the electroformed component from the mandrel.
Although the bath chemistry employed in electroforming is very similar to that of electroplating chemistry, the equipment and process requirements can differ considerably. While electrodeposits are used to enhance the surface properties of a substrate metal or nonconductor, electroforms are typically used as independent objects and are typically separated from the substrate mandrel after electrodeposition. Although good adhesion is a necessity in electroplating applications, separability of the electroform from the substrate mandrel is also essential for success in electroforming, and mechanical or metallurgical bonding of an electroform to its substrate mandrel would negate the purpose of the process.
Electroforming enables a user to manufacture complex shapes and surfaces at low unit cost and offers the ability to make shapes that would otherwise be impossible or impractical to mold in metal. Electroforming involves applying a coating to a three-dimensional shape, which enables items with very complex internal shapes, such as tubing manifolds, bellows, and mold recesses to be electroformed onto a machined or fabricated mandrel. Seamless objects, as well as complex shapes, which economically defy machining, can be repeatedly formed by electroforming. In addition, the nearly perfect surface reproducibility resulting from the electroforming process makes the process ideal for dimensionally exacting applications, including for example lens mold production, rotogravure printing plates, holographic embossing plates, and optimal memory disc mold cavities, among others.
An electroforming “mandrel” is the substrate or shape or form that the new electroform will take in the process. Mandrels are designed to be separated from the electroform and to be used again in the production process, and are typically made of a durable metal such as nickel, stainless steel or brass.
One application of electroformed copper is in the fabrication of copper cylinders, in which copper is plated onto a rotating stainless steel or other suitable cylindrical mandrel in a layer that is thick enough to be self-supporting and is then separated from the mandrel in order to form a finished cylinder.
There are several possible electrolytes for the production of copper electroforms including, cyanide copper, pyrophosphate copper and acid copper electrolytes such as sulfate and fluoroborate copper electrolytes. Most commonly, acid copper electrolytes are preferred and the copper sulfate/sulfuric acid electrolyte is the most widely used.
In order to produce electroforms of a suitable thickness, it is necessary to include additives in the plating electrolyte in order to prevent deposit nodulation, which would cause a deterioration in the mechanical properties of the electrodeposited copper. In the case of sulfate and fluoroborate electrolytes, the additives have typically included a combination of sulfopropyl sulfides and polyether molecules in the presence of chloride ions as described for example in U.S. Pat. No. 4,009,087 to Kardos et al. and in U.S. Pat. No. 3,778,357 to Dahms et al., the subject matter of each of which is herein incorporated by reference in its entirety. In addition other compounds may also be added as “leveling” agents to give copper deposits plated from the electrolyte scratch-hiding properties.
The inventors have discovered that oxygen in copper adversely affects copper's inherent high ductility, high electrical and thermal conductivity, resistance to deterioration when heating under reducing conditions, high impact strength, strong adherence of oxide scale, creep resistance, weldability and low volatility under high vacuum. In addition, there are applications for copper electroforms in which some welding of the fabrication is required. In this instance, the oxygen content of the copper electroforms must be low, typically below 10 ppm. However, copper electroforms produced on rotating cylinder mandrels often have high oxygen contents (up to about 500 ppm of oxygen).
The inventors believe that oxygen is incorporated into the deposit via two separate mechanisms. Firstly, the copper solution contains dissolved oxygen and the rotating cylindrical mandrel is often only partially immersed in the plating electrolyte. Thus, gaseous oxygen is in contact with the cylinder and may be subject to electrochemical reduction to form cuprous oxide, which is likely co-deposited at grain boundaries in the growing electroform according to the following reactions:2Cu2++2e−→2Cu+2Cu++½O2+2e−→Cu2O
The other mechanism by which oxygen can be incorporated into the deposit is by the incorporation of oxygen containing additives into the deposit. Additives modify the structure of the deposited copper by a mechanism of adsorption at growth sites, so some degree of incorporation of oxygen from the additives is inevitable.
Attempts have been made over the years to reduce the oxygen level in copper deposits. One prior art method of reducing the oxygen content of copper uses a remelting step under a controlled reducing atmosphere to produce a low-oxygen copper. This process has the disadvantage of being difficult to control. Another process involves deoxidizing molten electrically refined copper by the addition of a reducing material such as phosphorus, boron or lithium, producing the oxides of the metal and a low-oxygen copper. This process has the disadvantage of leaving dissolved reducing metal in the copper, which can adversely affect the properties of the copper. Another process involves the electroforming of low-oxygen copper from a mineral acid bath containing a wood such as Alleghany White Oak. This process has the disadvantage of being operable only at low current densities. Still another process involves the addition of a pentose, such as xylose, arabinose, ribose or lyxose to the plating bath, as described for example in U.S. Pat. No. 3,616,330 to Denchfield, the subject matter of which is herein incorporated by reference in its entirety.
However, there remains a need in the art for grain refining additives for copper plating baths which do not contain significant amounts of oxygen and for improved copper plating baths that are capable of producing electroformed copper deposits having low oxygen content.